This invention relates, in general, to deposition systems and more particularly to improvements in vacuum deposition systems that will insure a higher degree of uniformity of deposited material.
The typical production deposition system used for the deposition of either a layer of a single element or a layer of a combination of elements on a substrate consists of a vacuum chamber in which is contained the source of the material to be deposited and a substrate or target on which the source material is deposited. The system is usually placed on the vacuum and when the pressure within the vacuum chamber is reduced to the required level, the source material is heated to the point of vaporization and deposition on the target occurs.
In an attempt to insure a uniform deposition of source material in a production system which, for example, is required to deposit a layer of aluminum on a semiconductor wafer, as many as seven or eight wafer targets each having a three inch diameter are usually mounted on a disc or planet. A plurality of planets (usually three) are then mounted in rolling engagement on a pair of circular tracks so as to position the planets in a plane inclined toward the source. The source is usually located on the system axis and traditionally is positioned a few inches below the plane of the lower track on which the planets are rotating. The planets are then caused to rotate about the source, on the tracks, while the source material is vaporized. Thus, as each planet rotates about its axis each wafer target to be coated is caused to rotate about the axis of the planet and about the axis of the system.
In theory each of the wafer targets should be uniformly coated in order to achieve the best processing results. However, in actual practice, only the wafer target located on the axis of the planet will be uniformly coated while the wafers arranged around the planet, at its perimeter, will not achieve a uniform coating. This is based on the theory that since the density of evaporated material is inversely proportional to the square of the distance between the source and the wafer target, that portion of the wafer target closest to the perimeter of the planet will be exposed to a significantly higher density of material than its diametrically opposed portion. This phenomenon becomes more apparent when one considers the relative densities of evaporated material when the target is, for example, in the lowest portion of its travel, that is, when it's closest to the source as compared to the density of evaporated material when the wafer target is at the highest point of its travel, that is, when it is furthest from the source. The differences in density of material is reversed as the wafer target is rotated from its lowest point to its highest point about the axis of the planet. However, these differences in diffusion densities do not counteract each other and the net result is that the portion of the wafer target that is closest to the source when the wafer target is at the bottom of its travel will have a higher concentration of deposited material than the remainder of the wafer target. I have found, for example, a variation in coating thickness of as much as 3,000 Angstroms in a 12,000 Angstrom thick layer. Such variations in thickness across a wafer target having integrated circuits formed thereon can cause severe problems during the subsequent etching step.